ANALYSIS OF THE PERIODICITY OF SYNAPTIC EVENTS IN NEURONS IN THE SUPERIOR CERVICAL-GANGLION OF ANESTHETIZED RATS

1. The patterns of on-going synaptic events recorded intracellularly i n neurones of superior cervical ganglia (SCG) of anaesthetized female rats were analysed by constructing interevent interval histograms, aut ocorrelograms, ln-survivor curves and histograms triggered by the arte rial pulse wave and by the intercostal EMG. 2. In 11/12 cells with on- going frequencies >0.5 Hz, one or two inputs were strong (i.e, always suprathreshold). In five cells, action potentials also arose from syna ptic potentials with amplitudes close to threshold. 3. Synaptic events in 5/11 neurones tested were phase-related to the arterial pressure w ave (i.e. had cardiac rhythmicity, CR). 4. Synaptic events in 9/10 neu rones tested (including all with CR) were phase-related to the interco stal EMG and/or their autocorrelograms showed peaks at multiples of th e respiratory interval(i.e. had respiratory rhythmicity, RR). 5. The i ntervals between all synaptic events were exponentially distributed in 8/12 neurones although intervals between single strong events showed peaks related to the respiratory cycle. Bursts occurred only by chance . 6. Event patterns could be simulated by combining events from severa l respiration-modulated inputs with their timing distributed over near ly half the cycle. From the simulations, the mean number of active pre ganglionic inputs was estimated to be similar to 6 with mean discharge frequency similar to 0.4 Hz. 7. We conclude that, in the spontaneousl y breathing anaesthetized rat, most preganglionic neurones to the SCG fire with relatively low probability in relation to the respiratory cy cle. Rhythms in a postganglionic neurone reflect the activity of its s uprathreshold preganglionic inputs.